A laser ablation process generally includes selective removal of material at a surface of a workpiece by directing a laser beam at the workpiece. The laser beam is configured to deliver a controlled amount of energy at a laser spot defined where the beam impinges the desired surface. This controlled amount of energy is selected to liquefy, vaporize, or otherwise rapidly expand the surface material at the laser spot to cause it to separate from the workpiece for removal. Laser ablation can be used to remove at least a portion of one or more coatings from a coated substrate.
A conventional laser ablation process is typically performed at the focal height or distance of the objective (focusing) optics—i.e., with the focal plane of the laser beam at or near the surface from which material is to be removed. This gives the highest energy density and the smallest change in spot size with changes in the height of the surface. It has been found that with some conventional focused laser ablation processes, such as removing chromium (Cr) from glass with a picosecond green (532 nm) pulsed laser, the glass sustains surface damage to such an extent and with a regular periodicity that a diffraction grating is formed. Diffraction patterns that are produced by the diffraction grating can be an unwanted or unintended artifact of the ablation process.
The regular periodicity of the surface damage thought to be responsible for the diffraction grating is related to the laser pulse frequency and the scan speed. For example, at a 400 kHz pulse frequency and a 20 m/s scan speed, the spacing from pulse to pulse on the surface is 50 μm. A regular pattern of structures on a surface can generate diffraction according to the formula:d(sin θm+sin θi)−mλ, where d is the spacing of the pattern, θm and θi are the respective angles of the reflected and incident beams, m is the order of diffraction, and λ is the wavelength of light diffracted under those conditions. The diffraction observed for the laser ablated surface may require point light source illumination to be visible. The effect is pictographically shown in Figure A.
As depicted in Figure A, Ray A (center of grating surface) is reflected at an angle equal to its incident angle (specular reflection). An observer sees this as the reflection of the light source. Rays B and C are diffracted from the grating surface, and their incident and resultant angles are not equal. These rays may represent the first order diffraction for a particular wavelength. Because the observer is generally focusing on the image plane of the light source, the diffracted beams appear as spots or bars of color on both sides of the light source as depicted by D and E. For clarity, the example shown in Figure A demonstrates diffraction in one dimension, but actual diffraction artifacts may be multidimensional.
A microscope image of a laser ablated surface produced in an ablation process with a constant laser pulse frequency that may produce a diffraction pattern is shown in FIG. 2. Diffraction patterns produced by laser ablated surfaces may be objectionable to individuals observing the laser ablated surfaces.